A wireless communication system, in the form of a cellular system, is designed to cover a large geographic area. The system is divided into numerous cells providing air interface between mobile stations and land-based systems, such as the public land mobile network (PLMN). Recently, cellular based system design is used as a foundation for smaller, private systems, such as wireless office systems (WOS). These private systems may share the spectrum with the public cellular systems. Being smaller in scale than a public cellular system, a private cellular system is intended to operate at lower power levels.
Mobile stations are conventionally designed to adjust mobile station power level to keep the mobile transmitting at a high enough power level to minimize bit errors in the uplink direction. The balancing objective is to keep the mobile transit power low enough to minimize interference to other systems, and to extend the battery life of the mobile. In the public cellular systems the power level is controlled by commands in a digital control channel (DCCH). In wireless office systems the power level is set in control message commands sent to the mobile station. In either case the appropriate power level is determined by the wireless system and the mobile station controls its transmitter responsive to such commands. The power level used for private wireless systems is substantially lower than that used for public wireless systems. In fact the transmit power level in public wireless systems may be on the order of 100 mWatts, while for a WOS on the order of 1 mWatt.
The lower power level used for private wireless systems is an appealing feature for environments such as hospitals where any type of traditional cellular communications is strictly banned. The reasons for the ban include possible interference with hospital equipment such as heart monitors due to the high power level emitted from traditional public cellular system mobile stations.
As long as a private wireless system is operating normally, the power level from a mobile station will stay at low power level for office use. However, if the private wireless system experiences problems, there are possible failure situations where the mobile system will operate in the public network, and thus at higher power level, even if located within the area of the private wireless system. This can cause problems in sensitive environments, such as a hospital. One solution to this problem is to manufacture a mobile station that can only be used at a low power level, such as for hospital environments. The power level is then limited to never reach above the power level allowed in a hospital environment. However, this requires that a special mobile station be manufactured just for hospital use, resulting in a higher manufacturing costs. Administration costs would increase due to handling of non-standard phones. Further, the mobile station could never be used in a PLMN environment. Finally, one of the main features of wireless office system phones is the ability to use the phone both in the PLMN and the private wireless office environment, which would be removed.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.